This invention relates to a combination boiler and hot water heater which utilizes microwave energy to heat the water.
Conventional boilers and hot water heaters use either a combustible material or electricity as a source of heat. Combustibles derived from fossil fuels, such as natural gas, oil and propane, have numerous disadvantages, some of which are broad and some of which are specific to water heating applications. The broad disadvantages of combustion-based water heating systems include the limited and diminishing supply of fossil fuels, fuel price volatility, and emissions that contribute to air pollution and global climate change. In water heating applications, combustion-based systems involve risks of fuel leakage, explosion, fire, and contamination of the living space with dust and flue gas. Combustion-based systems are also expensive to purchase and install, occupy a lot of space, and require massive chimney structures to vent emissions. Moreover, such systems are characterized by low efficiencies and substantial heat losses.
Conventional electric boilers and hot water heaters, on the other hand, have their own set of problems. Generation of heat by an electrical resistive element is expensive, slow and inefficient. Electrical resistive elements used to heat water are subject to fouling with mineral deposits that limits their efficiency and their useful life and leads to high maintenance expenses.
In response to the deficiencies of conventional electrical and combustion-based systems, a number of microwave water heating systems have been developed and are described in the prior art. There are a number of generic advantages of such microwave-based systems as compared to their conventional counterparts. Microwave heating of fluids is rapid and efficient, thus addressing the need for energy conservation. There are no associated emissions and no need for elaborate and expensive ventilation equipment and chimney structures. Microwave boilers are relatively compact, inexpensive and easy to install. They are safe, involving minimal risks with respect to fire, explosion and leakage. They also require relatively little maintenance.
All of that being said, there still remain a number of problems associated with the application of microwave technology to conventional space heating and hot water systems. The fact that these problems have not been satisfactorily resolved by the prior art is demonstrated by the virtual absence of microwave boilers and water heaters in the marketplace. One of the problems with the prior art microwave systems is that they are not easily integrated with typical existing plumbing and heating systems.
The prior art systems also tend to be overly specialized. Some of these systems are designed to quickly provide hot tap water on demand, but are incapable of supplying large volumes of hot water, as would be needed to provide baseboard room heating. Other systems that do provide substantial boiler capacity are less efficient and responsive with respect to immediate hot tap water demands. Therefore, the prior art does not disclose a microwave domestic water heating system that effectively combines the functions of a boiler, to provide continuous baseboard room heat, and a hot water heater, to respond quickly when a hot water faucet is opened.
In terms of their structure, the prior art microwave hot water systems can be broadly classified as two types. The first type is a single plenum or “wet wall” design, in which the microwave source is mounted on a wall that is in direct contact with the water. In the single plenum design, the heater/boiler has only one interior space through which water flows. In some versions of the single plenum design, such as Brown, U.S. Pat. No. 3,891,817, the plenum comprises a tank of heated water, as in a conventional boiler. This type of system is adaptable to a hydronic space heating system, but is not well suited to supplying hot tap water. Other single plenum systems, such as Mayfield, U.S. Pat. No. 4,152,567, and Mencher, U.S. Pat. No. 5,247,148, have a low-volume cavity in which water is rapidly heated to supply hot tap water on demand. But these systems lack the storage capacity to provide hot water baseboard heat.
One of the problems with single plenum designs is uneven heating, causing liquid temperature gradients which reduce efficiency. The Mencher design attempts to overcome this problem by extending microwave dielectric plates across the entire length of the fluid flow chamber, but this has the disadvantage of limiting the size of the chamber and hence the volume of fluid heated. Another single plenum design, Moretti et al., U.S. Pat. No. 4,310,738, seeks to achieve uniform heating by using a microwave diffuser in combination with a network of upright metal baffle walls, which divide the plenum into a circuitous flow path. But in the Moretti design, the motor-driven diffuser consumes energy, making the overall system less efficient. Another drawback arises insofar as the upright baffle walls are imbedded in the plastic window through which the microwaves pass into the liquid plenum. The embedded metal in the microwave window gives rise to points of microwave reflection, in addition to problems associated with the metal-to-plastic seal, including potential leakage of microwaves, heat and water.
The latter problem of the Moretti design is one shared to some degree by all of the single plenum or “wet wall” systems. In these systems, the microwave source is directly coupled to the water-bearing plenum through a translucent microwave-permeable window, which protects the microwave source from water and steam that would damage its electrical components. The inherent weakness of the seams between this window and the metal wall enclosing the remainder of the plenum is a limiting factor on the temperature and pressure at which the system may safely operate. The limited integrity of the metal-to-glass or metal-to-plastic seal around the window also involves safety hazards with respect to microwave leakage and is a source of maintenance problems.
The problems associated with the single plenum or “wet wall” design have led microwave water heaters to evolve in the direction of dual plenum systems. In these systems, the heated water flows within a plenum which is enclosed inside a larger interior space to which the microwave source is coupled. Thus, the microwave source is coupled to a “dry wall” rather than a “wet wall”, thereby foregoing the need for a translucent window to protect the microwave source and eliminating the seam/seal problems associated with that window. In effect, therefore, there is a “wet” inner plenum, through which the heated water flows, and a “dry” outer plenum, to which the microwave source is coupled.
In most dual plenum systems, the “wet” inner plenum consists of pipe or tubing, typically having a helical or coiled configuration. Dual plenum systems of this type are disclosed in Martin, U.S. Pat. No. 3,812,315, Kaarup, U.S. Pat. No. 4,358,652, Matsuo et al., Pub. No. US 2002/0011487, and Jones et al., Pub. No. US 2005/0139594. Achieving uniform heating is, however, problematic in these systems. While the Martin system seeks to achieve uniform heating through a complex matrix of multiple dielectric plates and switches for applying sequential radiation to different areas of the coiled tubing, the overall system is large, cumbersome and expensive, with many potential breakdown points. Moreover, these dual plenum systems are all designed to deliver hot water on demand, and are not adaptable to supplying hot water baseboard heat.
Another version of the dual plenum design is disclosed in Stubbs, U.S. Pat. No. 4,114,001, in which the “wet” inner plenum comprises a parabolic array of microwave absorbent structures. In one embodiment of the Stubbs system (FIGS. 2 and 3), coiled piping serves as the inner plenum, similar to the Martin design, except for the parabolic configuration. In another embodiment of the Stubbs invention (FIGS. 5 and 6), the coiled piping is enclosed in a solid parabolic panel having multiple pyramid-shaped projections. In both cases, the heated liquid in the inner plenum is totally dedicated to a continuous flow through a system of heater exchangers or radiators for room heating (FIG. 1).
A comparison of the Stubbs and Martin patents points out a common deficiency in the dual plenum designs. Having segregated the heated water in an inner plenum tube or pipe, one can choose to use it either as a source of hot tap water or for room heating, but not for both at the same time. Another version of the dual plenum design attempts to overcome this limitation by using a chamber rather than a tube/pipe as the “wet” inner plenum, as disclosed in Riley, U.S. Pat. No. 5,387,780. In one embodiment of the Riley invention (FIG. 5), a once-through microwave-permeable coil can be inserted into the chamber/tank to provide hot tap water. But, since no mechanism in disclosed in Riley for circulating the water in the chamber around the coil to achieve even heat distribution, its ability to supply hot tap water at a constant uniform temperature is questionable.
It follows that the next logical step in the evolution of the microwave water heating systems will involve a triple plenum design. The present invention comprises a “dry” outer plenum, to which a microwave source is coupled, and two nested “wet” inner plenums. Comprising the two “wet” inner plenums are a bi-planar directional flow water jacket, which provides a continuous flow of hot water for baseboard room heating, and an adjustable sinuous length of heat exchange tubing, which supplies hot tap water on demand.